The present invention relates generally to a zoom lens system of a high variable magnification power and more particularly to a rear focusing type zoom lens system suited profitably for use in video cameras and the like.
As the prior art zoom lens system used in video cameras and the like, there is known one of such lens arrangement as disclosed in JP-A-63-133114 (and shown in FIG. 1 of the accompanying drawings) in which a first lens group located in opposition to the object and composed of three lens elements is moved along the optical axis for effecting the zooming operation.
Besides, there are known other structures of zoom lens systems in which the focusing is performed by moving a plurality or a few lens elements located closer to the plane of image formation. Reference may be made to JP-A-1-134413, for example.
In many of the zoom lens systems, driving means for moving the lens elements or lens group as mentioned above is constituted by an electric motor. In that case, it is preferred that the weight or load of the lens elements to be moved should be as low as possible because the motor can thereby be miniaturized correspondingly. In this conjunction, the prior art zoom lens systems mentioned above suffer from a problem that the weight of the lens elements to be moved for focusing and hence the load imposed on the motor is heavy because the focusing is accomplished by moving a lens group including three or more lens elements. An attempt for realizing the focusing by moving a single lens element has encountered difficulty in that the F-number has to be increased and the zoom ratio must be low in order to correct aberrations satisfactorily.